The present invention relates to a method of sensing current flowing through a MOSFET and, more particularly, to a current sensing method and device or the like suitable for a power conversion device using MOSFETs as switching elements.
In power conversion devices such as a DC/DC converter and a three-phase inverter, typically, currents flowing through switching elements are sensed and results of this sensing are reflected in control, and, for this purpose, current sensors of a magnetic field sensing type utilizing Hall elements have been used conventionally.
FIG. 9 shows a three-phase inverter device using MOSFETs as switching elements as an example of a power conversion device. The three-phase inverter is equipped with the current sensors of the magnetic field sensing type. As shown, this device has a main circuit 90 of the three-phase inverter to which a DC +/− voltage is supplied from an electrical source E such as a battery, and which supplies three-phase load L with three-phase AC power.
The main circuit 90 is comprised of a MOSFET 91 of a U-phase upper-arm, a MOSFET 92 of a U-phase lower-arm, a MOSFET 93 of a V-phase upper-arm, a MOSFET 94 of a V-phase lower-arm, a MOSFET 96 of a W-phase upper-arm, and a MOSFET 93 of a W-phase lower-arm.
These six MOSFETs 91 to 96 are each on/off controlled by switching signals that are fed from a gate driving circuit and work to convert the DC +/− voltage being supplied from the electrical source E into three-phase AC power, which has a predetermined voltage less than the voltage of the electrical source E and a predetermined frequency. The three-phase AC power is supplied to three-phase load L such as, for example, an induction motor.
Magnetic field type current sensors HU, HV, and HW using Hall elements are located on output lines of U, V and W phases of the main circuit 90. Current in each phase supplied from the main circuit 90 to the load L is sensed by the corresponding one of the current sensors. A sensed value of the current is input to a current sensing circuit, and sensing results from the current sensing circuit are input to a control circuit. Thereby, feedback control of the current by the control circuit can be accomplished.
By the way, the requirements for this current sensor include small size, low loss, and low cost as well as high accuracy; however, in general, a magnetic field type current sensor using a Hall element is comparatively large in size and costly.
Meanwhile, a current sensor using a shunt resistor has also been used conventionally, but, in this type of current sensor, loss proportional to current occurs and, consequently, the greater the current sensed, the loss will be a serious problem.
Then, a current sensing method which is illustrated in FIG. 10 has been proposed, wherein a sensing circuit 4 is connected to the source and emitter of a MOSFET 10 to take input of on-voltage of the MOSFET 10. The sensing circuit 4 converts the on-voltage into a current value, thus sensing a current flowing through the MOSFET 10.
Here, the on-voltage is a voltage developing between the source and drain of the MOSFET when the MOSFET is turned on by a gate signal and allowing current to flow through the MOSFET.
However, the on-voltage of the MOSFET strongly depends on temperature and changes in proportion to the square of an absolute temperature substantially. Therefore, the sensed voltage greatly changes with temperature change even if a constant current flows through the MOSFET.
FIG. 11 shows a graph of the varying ratio of on-voltage that the sensing circuit 4 takes at a constant current to the on-voltage when the junction temperature of the MOSFET 10 is 25° C., which is assumed to be 1. From this graph, it is seen that the on-voltage of the MOSFET strongly depends on temperature.
For this reason, in the case of the current sensing method illustrated in FIG. 10, thermal compensation is needed for the current sensor. For this purpose, a technique in which the MOSFET temperature is sensed and input to a microcomputer in the sensing circuit and the microcomputer computes a thermally compensated current value has so far been known (e.g., Japanese Patent Application Laid-Open No. 2003-61392).
FIG. 12 shows an example of an inverter device as a power conversion device to which this current sensing method is applied. This inverter dispenses with the magnetic field type current sensors HU, HV, and HW using the Hall elements, which exist in the inverter device shown in FIG. 9, and is arranged such that on-voltages from the sources and drains of the six MOSFETs 91 to 96 are input to the current sensing circuit.